The present invention relates to a semiconductor device, and more particularly to a protection circuit for electrostatic breakdown (it may be called as electrostatic discharge xe2x80x9cESDxe2x80x9d) in the semiconductor device including two or more kinds of power source systems in a single unit.
Now referring to the drawings, an explanation will be given of a semiconductor device according to a prior art. Many semiconductor devices, represented by a device incorporating a digital section and an analog section in the single unit, have two or more individual power source systems, which may be set at equal voltages.
An example of such a semiconductor device is shown in FIG. 3. As seen from FIG. 3, this device includes first circuitry which is an analog section composed of an analog circuit 1, an output circuit 3 and an input protection circuit 5, and second circuitry which is a digital section composed of a digital circuit 2 and an input circuit 4. Both sections are connected to each other by a signal line S1.
A power source voltage for the analog section is supplied from power source lines Vdd1 and Vss1, whereas a power source voltage for the digital section is supplied from power source lines Vdd2 and Vss2. The power source lines Vdd1 and Vdd2 are separated from each other, and the power source lines Vss1 and Vss2 are also separated from each other.
In the above device, an output signal is produced from the analog circuit 1 through the output circuit. The output signal is transferred to the digital circuit 2 of the digital section through the signal line S1. Specifically, the output signal is supplied to the input circuit 4 made of an inverter.
Because of any cause, where a surge input is supplied from an applying terminal NT which is an input terminal to the analog section, a rushing current SR occurs along the passage as shown in FIG. 3 so that the potential at the power source line Vss1 rises. Correspondingly, the potential in the signal line S1 also rises. This applies to the case where the surge input is supplied from the power source line Vss1.
In this case, the following problem occurs. The gate potential of a MOS transistor Q1 constituting an inverter of the input circuit 4 rises excessively so that the gate-source voltage of the MOS transistor Q1 increases excessively. As a result, the gate oxide film of the MOS transistor Q1 will be broken.
In order to prevent such electrostatic breakdown in the above circuit configuration, i.e. a semiconductor device in which a multiple power system having two or more kinds of power sources and first circuitry and second circuitry are contained therein and the first circuitry and the second circuitry are connected by a signal line, it is proposed to place a protection circuit (e.g. MOS transistor) between the first power source line Vss1 and the second power source line Vss2. In such a proposed circuit configuration, when the potential difference between the first power source line Vss1 and the second power source line Vss2 exceeds a prescribed value, the protection circuit becomes conductive so that both power source lines are made the same potential, thereby preventing occurrence of the electrostatic breakdown in the MOS transistor Q1.
However, in the above device configuration, the direction of the MOS transistor is determined based on which of the first power source line Vss1 and second source line Vss2 is larger or smaller than the other is determined. In a case where the direction of the MOS transistor is incorrectly set, the first power source line Vss1 and second power source line Vss2 are short-circuited to each other even if the protection circuit is provided.
An object of the present invention is to provide a semiconductor device capable of protecting electrostatic breakdown when a surge input occurs irrespectively of the magnitude relationship between the first power source line and second power source line.
According to the invention, a semiconductor device includes an N-type semiconductor substrate on which an analog section to which power source lines Vdd1 and Vss1 are supplied and a digital section to which power source lines Vdd2 and Vss2 are supplied. The analog section and the digital section are mounted and connected to each other by a signal line S11. The device also has a protection circuit HK1 located between the power source line Vss1 and Vss2. The protection circuit becomes conductive when a potential difference between the power source lines Vss1 and Vss2 exceeds a prescribed value irrespectively of the direction of the a surge input. Therefore, the power source lines Vss1 and Vss2 are placed at substantially the same potential.